Non-back-drivable gearboxes function to transmit torque from a high speed, low torque input shaft to a low-speed, high torque output shaft and does not transmit torque from the output shaft back to the input shaft. Conventional non-back-drivable power transmissions include worm gears and lead screws. Worm gearing is used for obtaining large speed reductions between non-intersecting shafts. The velocity ratio of worm gearing is the ratio between the number of teeth on the gear and the number of threads on the worm. The non-reverse drive characteristics of worm gearing relies on friction. It is required for non-back drive transmissions to have the power lost to friction equal to or greater than the output power. The theoretical maximum efficiency of a conventional non-back-derivable worm gearing is 50 percent.
Power transmissions having spur gears are disclosed in the prior art. W. J. Wallace Jr. in U.S. Pat. No. 2,178,443 discloses a non-reversible motion transmission mechanism having drive and driven shafts connected to planetary spur gears which are moved outwardly into engagement with a fixed ring spur gear when torque is applied to the driven shaft. J. A. Gilder in U.S. Pat. No. 2,995,226 discloses a reverse torque lock mechanism having a moving lock member that is forced into engagement with a fixed ring gear when reverse torque is applied to an output or driven shaft.